1. Field of the Invention
The present invention is related to an antenna array of a dual-antenna system, and more particularly to a two-dimensional antenna array and one-dimensional antenna array of a dual-antenna system having a high isolation.
2. Description of the Prior Arts
Generally, a radar transceiver has a receiving port and a transmitting port to respectively receive and transmit the radio signal. The radio signal can be transmitted to a farther way if an intensity of the radio signal is enhanced at the transmitting port. However, the radio signal at the receiving port is interfered by coupling with the radio signal with enhanced radio signal at the transmitting port. Accordingly, the radio signal of an entire middle frequency may easily enter a saturated state and a detection signal of a remote target is sunk in a noise signal. Alternatively, an intensity of local-port signal of the middle frequency may vary widely (e.g. a DC offset in a time domain response varied widely) and a quantization distortion is occurred when a digital signal processing unit processes the weaker detection signal of the remote target. Therefore, isolation means are respectively employed at an antenna port and a circuit of the radar transceiver to reduce a coupling interference between the receiving and transmitting ports.
There are two types of the isolation means. The first one is providing a single antenna with a circulator used in a conventional a radar system of a frequency modulated continuous wave. The second one is providing a dual-antenna structure. In the first type of the isolation mean, a largest isolation between transmitting and receiving ports is about −35 dB and an amplifier is not allowed to connect between the antenna and the circulator. In addition, an impedance of the antenna does not match that of the circulator, so that a reflection coefficient (S11) of the signal antenna is increased to result in a more signal leakage. Therefore, the isolation mean of the first type is not the best one and has other design limitation. With reference to FIG. 9, the dual-antenna structure 50 has a transmitting antenna 51 and a receiving antenna 52. Each of the transmitting antenna 51 and the receiving antenna 52 is a 4×1 one-dimensional antenna array 511, 521 consisting of four parallel single-stripe differential antenna units. A simple way to enhance the isolation between the receiving and transmitting ports is to increase a distance Dm between the receiving and transmitting ports. However, larger space is also required accordingly. Other types of the dual-antenna structure are further described as follows.
1. One type of the dual-antenna structure is a Branch Line Coupler: The branch line coupler exchanges a coupled factor and the reflection coefficient to have a high isolation and further uses a matching network to achieve a high isolation requirement.
2. An inductance element is added between the receiving and transmitting antennas of the dual-antenna structure, and an elongated slot or a T-shaped slot is formed on a grounding plane of the dual-antenna structure. The inductance element can be formed by adding metal plates or is a parasitic inductance. The modified dual-antenna structure provides a high isolation. However, a hole needs to be defined on the grounding plane to form the elongated slot or the T-shaped slot or a conductive via needs to be formed, and an antenna single end of a metal strip formed on an opposite plane also needs to be electrically connected to the grounding plane. A manufacturing procedure is more complex and the product uniformity is hardly controlled accordingly.
3. A slot is formed on a common grounding plane of the receiving and transmitting antennas so that a coupling current is restrained by a band-notched characterization and the isolation between the receiving and transmitting antennas is enhanced. However, this modified dual-antenna structure has drawbacks of forming the slot which are the same as the second modified dual-antenna structure mentioned above.
4. One type of the dual-antenna structure is a Dual Polarized Dielectric Resonator Antenna having two orthogonal polarized antennas and a T-shaped slot. The T-shaped slot reduces excitation mode coupling between two orthogonal polarized antennas to enhance the isolation of the two orthogonal polarized antennas. However, an accuracy of polarizing antenna is not easy and the isolation is not easily controlled. In addition, forming the T-shaped slot has the same drawback of the third type.
5. One type of the dual-antenna structure is a Leaky-Wave Dual Antenna System disclosed by TW invention patent No. I385857. With reference to FIG. 10, the leaky-wave dual-antenna system 60 has a transmitting antenna 61 and a receiving antenna 62. Each of the transmitting and receiving antennas 61 and 62 is composed of one-dimensional differential leaky-wave antenna array. To implement a differential feature, a signal phase difference between two feeding points of each of the transmitting and receiving antennas 61 has a 180 degree. A longitudinal length (L) of each of the transmitting and receiving antennas 61 and 62 is equal or greater than three times of a wavelength in a dielectric medium, so the leaky-wave dual-antenna system 60 can be excited to a high order mode and operates at high gain. Thus, the isolation of the leaky-wave dual-antenna system 60 is measured when the transmitting and receiving antennas are arranged in parallel and the isolation in a goal frequency range achieves −45 dB or more than that. The measured isolation of the leaky-wave dual-antenna system 60 is greater than that of the single antenna with the circulator. However, each of the transmitting antenna 61 or the receiving antenna 62 of the leaky-wave dual-antenna system 60 is not further designed as a two-dimensional antenna array since the longitudinal length thereof is equal or greater than three times of the wavelength in the dielectric medium and the one-dimensional antenna array has many design limitations. For example, a beam is only concentrated and shrunk at an azimuth direction based on a field pattern of the one-dimensional antenna array but the beam cannot be concentrated and shrunk at an elevation direction. Furthermore, a radiation angle at broadside of the leaky-wave dual-antenna system 60 is shifted 90 degrees when the leaky-wave dual-antenna system 60 operates at a high order mode. The applications of the leaky-wave dual-antenna system 60 are limited accordingly.
To overcome the shortcomings, the present invention provides a two-dimensional antenna array and one-dimensional antenna array of a dual-antenna system to mitigate or obviate the aforementioned problems.